I brought my MacBook to a friend in a different city this morning, he has the same internett provider I have. The emails came rolling in, no problems. I was about to call customer service now, but as I was waiting, I manually changed from my 5ghz wifi, to my 2.4ghz wifi. Instantly the mail app updated and even started downloading the 24000 or so emails that was now only on the server. Customer service from my internet provider had no clue why that worked, while 5ghz does not. At least it's a solution, but no real answer as to why. And Im stuck on my 2,4ghz network